- investigate the relationship between collision theory and reaction rate in order to analyse chemical equilibrium reactions (ACSCH070, ACSCH094)
Context Note:
Added in 2019, this critical dot point requires you to explain equilibrium shifts using Reaction Rate and Collision Theory, not just Le Chatelier's Principle (LCP). Questions specifying "collision theory" demand the rigorous logic outlined below.
- Question Types: After comparing reaction rates, questions may ask you to simply identify the shift (Option A) or fully explain the re-establishment of equilibrium (Option B). Both endings are provided in the templates below.
Scenario 1: Change in Concentration
Context: Iron(III) thiocyanate and varying concentration of ions (ACSCH095).
$$
\underbrace{Fe^{3+}{(aq)}}{\text{yellow}} + \underbrace{SCN^-{(aq)}}{\text{colourless}} \rightleftharpoons \underbrace{FeSCN^{2+}{(aq)}}{\text{blood red}}
$$
Disturbance: The concentration of Iron(III) ions ($Fe^{3+}$) is increased.
Method A: Le Chatelier's Principle (LCP) Approach
Use this for simple prediction questions.
Template Answer:
- Definition: Le Chatelier's Principle states that if a system at equilibrium is disturbed, the system will adjust itself to minimise the disturbance.
- Identify Disturbance: The concentration of reactant $Fe^{3+}$ has been increased.
- Apply LCP: In accordance with the principle, the system will shift to oppose the change by consuming the excess $Fe^{3+}$.
- Direction: The equilibrium position shifts to the right (products side).
- Result: The solution becomes a deeper blood red colour as $[FeSCN^{2+}]$ increases.
Method B: Collision Theory Approach (The "Hard" Explanation)
Use this for "Explain in terms of collision theory" questions.
Template Answer:
- Definition: Collision Theory states that reactions can only occur when particles collide with sufficient energy (greater than activation energy, $E_a$) and correct orientation.